The Chromebook screen doesn’t rotate in tablet mode.
Hit Ctrl > Shift > Refresh, or fix it in Settings.
One of the nice things about many Chromebooks is that you can fold the keyboard back (or detach it) and use it as a tablet. However, something may occasionally go wrong so that when you try to use it as a tablet and physically rotate it, the screen on your Chromebook won’t make the 90-degree rotation that you need. This can be really irritating — but there are a couple of very easy…
